Metallic Nanoparticles and Methods for Generating
Nanomaterials offer unique properties (e.g., magnetic, optical, mechanical, and electronic) that vary with changes in particle size. Metal-based nanoparticles such as Au, Pt, Cu, and Ag, and metallic oxides, for example, FexOy, have been used as industrial chemicals, catalysts, optical media, magnetic storage materials, materials for enhancing magnetic resonance imaging (MRI), and electrode materials.
However, metallic nanoparticles without a protective coating often have a high propensity to oxidize or undergo other chemical reactions. Metal-core carbon-shell nanoparticles (sometimes referred to as “carbon-encased metal nanoparticles”), which have a metallic core surrounded by a carbon shell, can broaden the uses of metallic nanoparticles. The metal in such nanoparticles is protected against chemical reactions, and the carbon shells may be functionalized to have specific physical, chemical, and biological properties.
Currently, synthesis of metal-core carbon-shell nanoparticles is based on wet chemical or expensive physical methods that would be difficult to scale for commercial applications. The common techniques to make these and other types of nanoparticles are described below.
Metal Evaporation is a simple way to fabricate metal/metal oxide nanoparticles. A mixture of a metal and its oxide is placed in a basket or pouch through which an electrical current is passed. This process causes the metal and its oxide first to melt and then to vaporize. An electron beam may be used to assist in vaporizing metal/metal-oxide mixtures having high melting temperatures. Vaporization may occur either in vacuum or in an inert gas. Vaporized metal/metal-oxides are solidified directly on a substrate placed above the basket or pouch. The size and size distribution of such particles depend on a number of parameters including whether they are generated in an inert gas or vacuum, and if in a vacuum, the pressure. Metallic nanoparticles produced by this method tend to be very reactive, which is useful for some applications but undesirable for others.
Sonochemical processing is a method for generating nanoparticles in an ultrasonicated solvent. Sonochemistry involves acoustic cavitation, which appears to be caused by an implosive collapse of a bubble in an ultrasonically irradiated liquid. This process generates a transient, localized hot spot with an effective temperature of about 5000° K at pressures of about 1000 atm. Heating and cooling rates may be greater than 1000° K/s. Acoustically cavitated bubbles produce large pressure variations and fluid motion. In addition, there may be other sonochemical effects, such as momentum and mass transport, generation of radicals and other excited particles, formation of high velocity liquid jets, and generation of shockwaves external to the bubble. While metallic nanoparticles may be made by this method, the process is difficult to control.
Chemical Reduction of metal ions in solution may be used to form metallic nanoparticles. This process begins with a solution of a metal ion. A reducing agent added to the solution causes precipitation of the metal, metal alloys, or metal carbides. Poly-alcohols, such as ethylene glycol or diethylene glycol, have been used as both solvent and reductant. Particle size of precipitants depends on the rate of nucleation and growth, and may be affected by aggregation during growth. This method has been used to form Au, Cu, Te, and Pt nanocrystalline metals, and nanocrystalline intermetallics.
Protective Layers. Because of their high surface area-to-volume ratio, many metallic nanoparticles, including copper nanoparticles, are prone to readily oxidize or otherwise chemically react. To avoid this problem a protective layer may be formed around the nanoparticles. Several techniques, such as layer-by-layer assembly, formation of microemulsion, modifications of the Kratschmer-Huffman carbon arc method, hydrocarbon decomposition, arc discharge in de-ionized water, hydrolysis of tetraethoxysilicate (TEOS), plasma polymerization, plasma torch synthesis, and flow-levitation have been used to coat metallic nanoparticles. Some of the more common methods are described below.
Layer-by-Layer Assembly (LBL) is a method for fabrication of composite nanomaterial films. This method is based on the sequential adsorption of substrates in solutions of oppositely charged compounds. Nanometer film thicknesses are typical. Deposition is controlled by adjusting processing conditions, such as solution pH, ionic strength, and immersion time.
Microemulsion is a method in which two or more immiscible substances are dispersed. This dispersion or mixture typically contains water, oil, a surfactant, and sometimes a co-surfactant. An oil-in-water (O/W) microemulsion is one in which oil is at a droplet center surrounded by surfactant and co-surfactant. A water-in-oil (W/O) microemulsion is one in which water is at a droplet center surrounded by oil. The submicroscopic droplets/micelles may take up solutes and often exhibit different environments than exhibited by bulk solvents. Microemulsion droplets have been used to encapsulate water-soluble agents such as nanoparticles and submicron particles.
Kratschmer-Huffman Carbon Arc is a method originally used to make fullerenes. The method uses graphite rod electrodes to produce a continuous DC electric arc discharge in vacuum. Carbon that evaporates from an anode produces carbon soot. In high vacuum, the method produces a hard, graphite-like substance, while at lower vacuum it forms a fine soot rich in fullerenes and other nano-materials. This method has been modified by simultaneously evaporating carbon and a metal to generate nanoparticles comprising a carbonaceous material that encapsulates a metal nanoparticle. However, it appears that this process often results in the metal being oxidized.
Typically the existing methods for generating core-shell or coated metal nanoparticles are either too selective or too difficult to implement. For example, the LBL method has been reported to date to be limited to polymer and coated noble metal nanoparticles.
The microemulsion method cannot be applied generally because it requires very specific recipes for each metal salt. Further this method is limited by the reducing potential of the metal salt. In addition, nanoparticles generated by the microemulsion method tend to agglomerate. Further, like the LBL method, this method has been reported to date only to produce polymer or coated noble metal nanoparticles.
While the Kratschmer-Huffman carbon arc method appears to produce high quality carbon shells on metal particle surfaces, it also produces numerous by-products, nanoparticles with large variations in shape and size, and some metal oxides.
In addition, none of the existing methods for generating metallic nanoparticles routinely achieve complete carbon coating which provide complete protection of metallic nanoparticles. The metallic nanoparticles tend to oxidize readily in air from the exposed surfaces, resulting in a short shelf-life for these materials unless stored under an inert gas. Scaled-up production using any of these methods also will be difficult and expensive, because precise control of the method variables is difficult.
J. He et al., “Facile synthesis of noble metal nanoparticles in porous cellulose fibers,” Chem. Mater., vol. 15, pp. 4401-4406 (2003) reported in situ, wet-chemistry formation of Ag, Au, Pt, or Pd nanoparticles in porous cellulose fibers from the corresponding noble metal salt precursors, using a traditional reducing agent (NaBH4). This method was not reported to produce a carbon shell.
J. He et al., “Facile fabrication of composites of platinum nanoparticles and amorphous carbon films by catalyzed carbonization of cellulose fibers,” Chem. Commun., Issue 4, pp. 410-411 (2004) reported carbonizing cellulose matrixes containing reduced platinum nanoparticles. J. He et al proposed that the platinum nanoparticles may have catalyzed the carbonization of the cellulose matrix. The resulting product consisted primarily of amorphous carbon fibers and Pt nanoparticles.
H. Zhu et al., “Synthesis of assembled copper nanoparticles from copper-chelating glycolipid nanotubes,” Chem. Phys. Lett., vol. 405, pp. 49-52 (2005) reported an annealing process for assembling copper nanoparticles from copper-chelating amphiphiles using glycolipid nanotubes.
Raymond et al, “In-Situ Synthesis of Ferrites in Cellulosics,” Chem. Mater. 6:249-255 (1994) disclosed that nano-ferrite particles could be generated within a cellulosic matrix. This method appears to use an ion-exchange mechanism to retain these particles within the cellulosic matrix. No carbonization of this material was reported.
There exists an unfilled need for metallic nanoparticles that are stable in air and water, while still exhibiting desirable chemical activity, and a method of generating such metallic nanoparticles that is cost efficient and scalable for industrial use.
Wood Preservation
Millions of homes are constructed each year from light frames made of wood. Wood, unless protected, is naturally degraded by heat, moisture, insects, decay, mold and other forces. Formosan subterranean termites (Coptotermes formosanus) can be particularly destructive to wood.
It is well known that copper is a very effective wood preservative. In recent years, primarily because of environmental concerns, the most widely used form of copper, chromated copper arsenate (“CCA”), has been substantially reduced. Removal of CCA from the market has made wood preservation more difficult.
Laks et al, U.S. Pat. No. 6,753,035 disclosed a method for incorporating additives such as biocides into wood or wood products using polymeric nanoparticles.
Richardson et al, PCT Application WO 2006/065684 disclosed methods for protecting wood against insect attack and decay by injecting sparingly soluble copper hydroxide-containing particles into wood and wood products. This disclosure taught the use of micron size particles, and suggested that particles smaller than 0.02 microns would tend to convert from metallic copper to copper oxide and be easily flushed from the wood.
Copper-containing wood preservatives that have been proposed to replace CCA typically use one or more soluble copper ions (e.g., Cu++). Copper complexes such as copper alkanolamine complexes, copper polyaspartic acid complexes, alkaline copper quaternary salts, ammoniacal copper quaternary salts, ammoniacal copper zinc salts, copper azole, copper boron azole, copper bis-(dimethyldithiocarbamate), ammoniacal copper citrate, copper citrate, and copper alkanolamine carbonate complexes have been suggested. However, due primarily to cost, the only formulations that have been used commercially are copper alkanolamine complexes and copper ammonium complexes.
Wood is naturally resistant to mildews and certain molds, in part because there is very little fixed nitrogen in wood. However, amine and ammonium complexes that add nitrogen to the wood may paradoxically promote mildew or mold. For example, amine and ammonium copper complexes appear to facilitate increased sapstain mold formation and enhanced mildew formation.
Another problem with commercially available copper-based preservatives is that they tend to be water-soluble, making them subject to leaching from wood exposed to moisture. To maintain protective amounts of copper in wood, a higher concentration of copper may need to be impregnated into the wood. While such an approach does not prevent leaching, it does increase the time during which the copper is effective. However, since the copper ions are thought to be toxic to aquatic life, such an increase in the copper loading is not desirable because of the associated increase in copper ions discharged into the environment. In addition, increasing the amount of copper increases the cost of this process. Also, because amine and ammonium complexes may emit vapors, large amounts of these complexes of copper tend to increase the odor and irritation from the amine and ammonia fumes.
Another problem with existing commercial, copper-containing wood preservatives is that they often cause corrosion of metal fasteners and other hardware. Metal ions as well as amines, alkanolamines, and ammonia used in soluble copper treatments appear to contribute to corrosion of metal hardware. Thus, commercial copper-based wood preservative may not be suitable for outdoor wooden structures, unless galvanized metal or stainless steel is used for all fittings. Use of such hardware will make the wooden structure more costly.
An unfilled need exists for copper-containing wood preservatives that are economical, that do not readily leach, that do not emit noxious vapors, and that do not cause corrosion of fasteners.
Strengthening Polymer and Polymer Composite Materials
Polymers offer many advantages over conventional building materials including lightness, resistance to corrosion and ease of processing. Polymers may be used alone or in combination with fibrous materials. In addition, polymers may be used as additives to form composites, which may be used as structural members. Polymer composites can be used in many different forms ranging from structural composites in the construction industry to the high technology composites of the aerospace and space satellite industries.
In recent years, composites comprising natural fibers, for example wood, and reinforced plastic have become one of the most rapidly growing markets within the polymer industry. In some markets more than 80% of products such as decking, railing, windows, door profiles, and shingles are either polymeric or fiber/polymer composites. Other uses of these materials include infrastructure, for example boardwalks, docks, and related structures, in the transportation industry, in automobiles, for example interior panels, rear shelves, and spare tire covers, and within the industrial/consumer industry, for example picnic tables, park benches, pallets, and other similar products.
However, some concerns over product quality and product toughness remain. Using wood or other natural fiber as filler in composites increases composite stiffness, but appears to reduce the toughness of the composite. Brittleness appears to be caused in part by stress concentrations at fiber ends and by poor interfacial adhesion.
Thus an unfilled need exists for new additive or coupling agents for composites that improve toughness while maintaining or improving other composite properties.